Seems like Apple isn't letting the Library of Congress's declaration that jailbreaking is legal stop it from throwing further obstacles in the way of people trying to open up their iOS hardware. The latest such gimmick is iBooks. Version 1.2.1 of the app, launched last week, refuses to open books on jailbroken phones, as detailed by Social Apples.
Comex of the iPhone Dev Team explained how the security measure works on Twitter. It seems that, before loading any DRM-controlled book (i.e., anything you buy from the iBookstore), iBooks tries to run a small program that has deliberately not been correctly signed to work on the iOS device. Normal devices will refuse to run this; iBooks detects that failure and loads as usual. However, jailbroken devices will (by default) run the program fine, which iBooks also detects and refuses to go any further.
UPDATE: further investigation suggests that, rather than this being new protection in the latest iBooks release, the protection itself is unchanged. Cydia developer Pushfix has written several posts on their Twitter account that clarify the situation. iBooks has always used a number of different checks, but under older jailbreak apps and iOS versions, many of the checks have passed despite the jailbreak. However, newer jailbreak routines have lifted more and more different restrictions that iOS applies internally, meaning iBooks has (to the user) appeared to become better and better at detecting the presence of a jailbreak.
Presumably, Apple took this step to protect iBooks content from having the DRM stripped off through use of a jailbroken device. Of course, people wanting to pirate e-books have plenty of places to do so; like so many antipiracy measures, this did a lot more to annoy paying customers than it did to slow down the pirates.
In any event, as is often the case in these cat-and-mouse games between Apple and the jailbreak devs, it wasn't long before version 0.9.5b5-4 of redsn0w was released to fix this issue . UPDATE: as numerous commenters have spotted, that link goes to a release of redsn0w from the middle of 2010 that fixed a similar issue with iBooks but does not address this latest change by Apple. At present, there is no solution short of removing the jailbreak altogether, though Comex reports that he has a fix ready. We apologize for the error. UPDATE2: several hours later, the iPhone Dev Team obliged us with PwnageTool 4.2 with a fix for the iBooks problem.
[Thanks to Josh for sending this in, and Rene Ritchie for prompting us to dig deeper into the background issues.]